The ability to transfer heat from the voice coil is a basic design limit of high power loudspeakers. Magnets and plates surrounding the voice coil are not made of high thermal conductivity materials. The insulation wrapping the voice coil wire is by definition insulation, and barely conducts heat to the air surrounding the voice coil. The air layer around the voice coil is almost an ideal insulator. This doubly insulated voice coil works in constrained air spaces that quickly rise in temperature. This rising temperature in the neighborhood of the coil aggravates the increasing temperature of the voice coil wire and the associated efficiency loss of the voice coil. Efficiency of the coil could be increased by adding additional layers of winding to the coil, but this results in additional heating of interior coil wire windings where the coil wire is surrounded on three or four sides by hot coil wire and has to rise above the temperature of its neighborhood coil wire to dissipate the heat generated by the current it carries.
Eventually every loudspeaker meets its threshold where an increase in voltage causes enough coil resistance increase so the current in the coil decreases and the loudness of the loudspeaker goes down. See U.S. Pat. No. 5,664,023. One solution to this problem would be to increase the room temperature efficiency of the speaker, but this has not worked well for woofers where attempting to increase the efficiency in a given size structure causes a decrease in low frequency performance.
Furthermore, a high power, high efficiency full range, flat frequency response single diaphragm loudspeaker that images as an ideal point source is a very difficult objective when all the other objectives listed above are to be simultaneously met. Meeting this ideal point source requirement means being an omni directional source with no phase shifts as a function of radiated angle or distance.